Theoretical investigations of oxygen vacancy effects in nickel-doped zirconia from ab initio XANES spectroscopy at the oxygen K-edge

Abstract

In this study, we present theoretical X-ray absorption near-edge structure (XANES) spectra at the K-edge of oxygen in zirconia containing Ni dopant atoms and O vacancies at varying concentrations. Specifically, our model system consist of a supercell composed of a zirconia (ZrO₂) matrix containing two nickel dopants (2Ni), which substitute two Zr atoms at a finite separation. We found the 2Ni atoms to be most stable in a ferromagnetic configuration in the absence of oxygen vacancies. In this system, each Ni atom is surrounded by two shells of O with tetrahedral geometry, in a similar way as in bulk cubic zirconia. The oxygen K-edge XANES spectrum of this configuration shows a pre-edge peak, which is attributable to dipole transitions from O 1s to O 2p states that are hybridized with unoccupied Ni 3d states. The intensity of this pre-edge peak, however, reduces upon the introduction of a single vacancy in the 2Ni-doped zirconia matrix. The corresponding ground state remains ferromagnetic, while one of the nickel atoms adopts a trigonal bipyramidal geometry, and the other one remains in a tetrahedral geometry. Furthermore, the introduction of two vacancies in the 2Ni-doped zirconia results in the two Ni atoms having distorted octahedral and trigonal bipyramidal geometries and being coupled antiferromagnetically in the ground state. Additionally, the oxygen K-edge XANES spectrum shows a further decrease in the intensity of the pre-edge peak, compared to the case of a single vacancy. Thus, the changes in the intensity of the pre-edge peak evidence a major structural change in the local environment around nickel atoms and, by extension, in the zirconia matrix. This change is due to the structural disorder induced by the 2Ni dopants and the O vacancies. Furthermore, the analysis of the XANES signatures shows that the oxidation state of nickel atoms changes with the introduction of oxygen vacancies. Our study therefore shows a possibility to control the oxidation state and magnetic order in a typical diluted magnetic oxide. Such a finding may be crucial for spintronics-related applications.

Keywords: X-ray absorption; X-ray absorption near-edge structure (XANES); defect; ligand field; nickel; oxidation state; oxides; spectroscopy; spintronics; vacancy; zirconia.

Figure 1

(a) Zirconia containing two Ni atoms substituting Zr atoms without O vacancies, that is, structure S₀. (b) Structure S₁, which is S₀ containing a single O vacancy wherein the oxygen atom (green sphere) marked X1 in (a) has been removed. (c) Structure S₂, which is S₀ containing a double O vacancy wherein the O atoms (green spheres) marked X1 and X2 have been removed.

Figure 2

Relaxed nickel geometries and crystal-field splitting diagrams for different Ni-doped zirconia containing varying oxygen vacancy concentrations. (a, b) Nickel dopant atoms in tetrahedral geometry in the structure S₀ with Ni1 and Ni2 being spin-up polarized. It should be noted that S₀ has no oxygen vacancies. (c, d) Nickel dopant atoms in trigonal bipyramidal and tetrahedral geometries in the structure S₁ with Ni1 and Ni2 being spin-up polarized. Structure S₁ has one oxygen vacancy. (e, f) Nickel dopant atoms in octahedral and trigonal bipyramidal geometries in the structure S₂ with Ni1 being spin-up and Ni2 being spin-down polarized. Structure S₂ has two oxygen vacancies. In the figures, blue and red spheres represent Ni and O atoms, respectively. Black filled arrows represent occupied electron states while the hollow arrows indicate empty electron states.

Figure 3

(a) O K-edge spectra of pure zirconia (blue curve) and of nickel-doped zirconia (ZrO₂:Ni) at x = 6.25 atom % Ni concentration and varying number of vacancies. The notations S₀, S₁, and S₂ correspond to the doped structures containing zero, one, and two oxygen vacancies, respectively. Note the presence of the pre-edge peak, which decreases when the number of oxygen vacancies increases. Also, the black, red, and green curves are the XANES spectra obtained from the averages of 64, 63, and 62 individual O K-edge spectra in the zirconia supercell, while the spectrum E (black dashed line) corresponds to the experimental O K-edge spectrum in iron-doped zirconia (ZrO₂:Fe) at x = 6 atom % Fe concentration from [27]. (b) The mean contributions of the first oxygen shells around the nickel atoms Ni1 (solid lines) and Ni2 (dashed lines) to the O K-edge spectra of the structures S₀, S₁, and S₂.

Figure 4

Projected densities of states (PDOS) of nickel atoms Ni1 and Ni2. The solid line corresponds to spin-up polarization, the dashed line corresponds to spin-down. (a, b) Nickel atoms Ni1 (spin-up) and Ni2 (spin-up) in tetrahedral geometries, belonging to the doped structure S₀. (c, d) Nickel atoms Ni1 (spin-up) and Ni2 (spin-up) in trigonal bipyramidal and tetrahedral geometries, respectively, belonging to the doped structure S₁. (e, f) Nickel atoms Ni1 (spin-up) and Ni2 (spin-down) in octahedral and trigonal bipyramidal geometries, respectively, belonging to the doped structure S₂.